。
She's by far the tallest girl.
Literal
She [topic-は] standing-out the-most back [subject-が] tall girl is.
ずば抜けて ('by far / standing out conspicuously') is the te-form of 飛び抜ける/ずば抜ける ('to stand out far above') used adverbially. The compound verb breaks down as ずば (intensifier) + 抜ける ('come out / pass through'). Layered on top of 一番 ('most / number one' — the standard superlative marker), the doubled emphasis makes the contrast extreme: not just 'the tallest', but 'tallest by a wide margin'. 背の高い ('tall' — literally 'one whose back is tall') is the standard idiom for height; the の inside is a small relative-clause-style modifier where the subject 背 takes の instead of が.